Explosion suppression system including explosion simulation and testing apparatus



June 24, 1969 c. F. ROCKWELL 3,452,350 EXPLOSION SUPPRESSION SYSTEMINCLUDING EXPLOSION SIMULATION AND TESTING APPARATUS Filed April 1966Sheet I of 3 J z w? N g C 0 P a u. s aa w I) 5?.1 all! .4 Tm i -l PE 09- 0 "Q N 5 04? um U a g 5- E "'P U U W4! P 5 cs m o D 0 z z 2 I a 5 w Ea 2% n 6 U y t; U m k 2 w 33 09 P g .IO F 2 w m '||||l|', A O O 52 2 r S2 2 g 9 "5o 30) INVENTOR.

CHARLES F- ROCKWELL BY ,M m

1 ATTORNEYS June 24, 1969 c. F. ROCKWELL 3,452,350

EXPLOSION SUPPRESSION SYSTEM INCLUDING EXPLOSION SIMULATION AND TFS'ITNG APPARATUS Filed April 5. 1966 Sheet 2 of s an m 0 J -D 43 Z a N Q 2a 5 k h S2 P a m 0 11..

JJ w- N L|: .N 4: e g o 4:

u z O z u. w o o a 8 .1 o O a o o g 2 E2 0 1 (Q P P P: .1: (fl 3 .u m Nu E i o w w cnu Z Z I JNVENTOR. CHARLES F. ROCKWELL ATTORNEYS June 24,1969 c F. ROCKWELL 3,452,350

EXPLOSION SUPPRES SION SYSTEM INCLUDING EXPLOSION SIMULATION AND TESTINGAPPARATUS Filed April 5, 1966 Sheet of 3 FIG, 3

U) I a I I hgllp I 5' $3 PJ l- N m H .J a m U 7 MW I E I a 1 I w v--- II- CHARLES Rama BY I L? Q LHLC KUZL ATTORNEYS 3,452,350 EXPLOSIONSUPPRESSION SYSTEM INCLUDING EXPLUSION SIMULATION AND TESTING APPARATUSCharles F. Rockwell, Sherborn, Mass., assignor to Fenwal, Incorporated,Ashland, Mass, a corporation of Massachusetts Filed Apr. 5, 1966, Ser.No. 540,318 Int. Cl. G08b 29/00 US. Cl. 340-410 4 Claims My inventionrelates to explosion suppression, and particularly to a novelself-testing explosion suppression system including apparatus forsimulating an explosion and testing the suppression system withoutfiring the suppressors.

Explosion suppressors have been developed for preventing an explosiverise of pressure following. the ignition of a combustible mixture ofgases in an enclosed or partially enclosed container. These suppressorscomprise a frangible container of inerting fluid, such asdibromotetrafluoroethane, and an electrically detonated charge forbursting the container and expelling the inerting fluid into the burningmixture before the combustion can result in a destructive rise inpressure.

An incipient explosion in a space may be detected by radiationresponsive means, such as a photovoltaic cell, responding to radiationfrom a flame front to actuate the suppressor before a dangerous rise inpressure can occur. Illumination of the photocell by radiationcharacteristic of a flame actuates a circuit that supplies current tothe firing circuit of the suppressor. When the suppressor is fired, itsactuating circuit is broken.

In my co-pending US. application Ser. No. 540,318 filed on the same dayas the present application, for Control and Indication System forExplosion Suppressors and assigned to my assignee, I have disclosed acontrol circuit for explosion suppressors in which a switch is includedin the circuit for supplying current to the suppressors that is openedas soon as the suppressor ignition circuit is opened. By thatarrangement, electrical voltage does not appear across the terminals ofthe suppressor after a suppressor has been actuated. The apparatus of mycopending application also includes indicating means for visuallyindicating the actuation of an explosion suppressor, so that it will bereplaced, and not relied upon for protection, after it has been used.Although that apparatus does provide a clear indication after the factof the operation of the suppressor, it would be highly desirable to beable to ascertain the oeprativeness of the apparatus at any time withoutactuating the indicator and suppressors, so that a protection systemincluding an open circuit, a short circuit, or a defective component,would not be relied upon for protection. It is the object of myinvention to enable the complete check of an explosion suppressionsystem, without more than a temporary interruption of its normalstand-by operating condition, and without reducing the safety andreliability of the system.

Briefly, the apparatus of my invention is built upon the basiccombination of a source of voltage, an electronic switch, and acurrent-interrupted circuit connected in series, together with acondition-responsive signal generator for controlling the switch toclose it in response to a predetermined condition and send currentthrough the current-interrupted circuit to perform a desired function,such as the ignition of an explosion suppressor or the like. Inaccordance with my invention, I include in this series circuit a secondswitch that is normally closed. Connected in parallel with the secondswitch is a capacitor and a current limiter connected in series. Ifurther provide sequencing circuits operating in response to a testcommand signal to open the second switch and then supply a nited StatesPatent ice simulated command signal to the signal generator to cause thefirst switch to be closed. The capacitor will thereby be charged throughthe current limiter. The sequencing circuits next open the first switch.Finally, the second switch is again closed, to discharge the capacitorthrough an indicating circuit that will respond only if the limitedcurrent has been passed by the basic circuit and the second switch isoperating properly. By this arrangement, both circuit continuity andproper sequence of operation of all circuit components are checked. Aswill appear, the apparatus is automatically restored to its originalstate following the completion of a successful test.

The apparatus of my invention will best be understood in the light ofthe following detailed description, together with the accompanyingdrawings, of a preferred embodiment thereof.

In the drawings:

FIG. 1 is a schematic block and wiring diagram of a control and testingcircuit for a current-interrupted element comprising thecondition-simulating and circuit testing apparatus of my invention;

FIG. 2 comprises a timing chart illustrating the operation of thecircuit of FIG. 1; and

FIG. 3 is a schematic wiring diagram of an explosion control systemcomprising a specific embodiment of the apparatus of FIG. 1.

In FIG. 1, I have shown a circuit arranged to supply a pulse of currentto a current-interrupted actuating bridge wire W, as for an indicator,explosion suppressor, or the like, in response to a command signalsupplied to a signal generator SG. The basic actuating circuit extendsfrom ground through a suitable source of voltage, here shown as abattery B, across the terminals of a normally open switch S1, throughthe bridge wire W, and through a diode D1 and a normally closed switchS2 to ground.

The switch S1 is arranged to be closed by a switch control SCI, eitherby an output signal produced by the signal generator SG, in response toan applied command signal or a simulated command signal. The commandsignal may be any condition which it is desired to detect, such as theradiation from a flame front. The simulated command comprises a pulse ofthe same nature as the command signal, produced by the sequencingcircuits SQ in response to a test command signal such as the depressionof a pushbutton.

The sequencing circuits SQ also supply an operating control signal to aswitch control SC2 for opening and closing the switch S2, supply a testreference voltage to a current limiter CL, and supply an indicate enablelevel to a test indicator TK in response to the test command level. Aninhibit signal is applied to the sequencing circuit SQ from the junctionof the bridge Wire W and the diode D1. The inhibit signal comprises aground level current sink preventing operation of the sequencingcircuits SQ to produce a simulated command signal while the switch S2 isclosed. As will appear, that circuit prevents the operation of the testcircuits to supply current to the bridge wire W if there are any shortsto ground in the firing circuit.

A capacitor C1 and a current limiter CL are connected in series, and theseries combination is connected in parallel with the switch S2. Thecurrent limiter CL may simply comprise a resistor, but in the preferredembodiment also includes means controlled by the test reference signalsupplied by the sequencing circuits SQ for limiting the voltage acrossthe capacitor C1 in the test mode of operation.

As shown, a diode D2 is connected in series with a test indicator TKbetween ground and the junction of the capacitor C1 and the currentlimiter CL. When supplied with an indicate enable signal from thesequencing cir- 3 cuit SQ, with the switch S1 open, the test indicatorwill be operated by the discharge of the capacitor C1 through the diodeD2 if the capacitor has been charged.

The operation of the apparatus of FIG. l is illustrated in FIG. 2. InFIG. 2, signals are indicated as 01f or on, switches are indicated asopen or closed, and voltages are indicated as at volts, for groundpotential, or above ground. Sequential times during the operation areindicated by sequentially indexed symbols ti, as indicated in FIG. 2. At10, the switch S2 is closed, the switch S1 is open, the voltage Vcacross the capacitor C1 is 0, the indicator TK is 011", the signallevels test command and simulated command are ofi, and the level inhibitis on. At some time t1, the test command level is produced and will stayon until some later time t10 at the end of the test.

At a very short time t2 later than 11, the switch S2 is opened, and thepreviously present inhibit signal goes off. At a predetermined time 13later than [2-, the sequencing circuits Sq will produce a simulatedcommand signal pulse applied to the signal generator 56.

At a time 14 slightly past t3, the signal generator SG will produce anoutput signal closing the switch S1 to permit a limited current flowfrom the battery B through the switch S1, the bridge wire W and thediode D1, charging the capacitor C1 through the current limiter CL. Thecurrent limiter CL is selected to produce a current that will notinterrupt the bridge wire W. As indicated in FIG. 2, the time t5 atwhich charging begins is substantially t4.

Comparing FIGS. 1 and 2, as the capacitor C1 charges the voltage Vc willrise. At some time t6 after [3, determined by the operation of thesequencing circuits SQ, the simulated command signal will go off". Theon time of this signal is selected simply to be long enough to operatethe signal generator SG and short enough to be Well ahead of the time I8when the switch S2 is closed. The time t7 when the switch S1 is openedmay be caused to occur by timing or voltage measuring means controlledby the voltage across the capacitor C1, or by the closure of the switchS1, in the broader aspects of my invention. However, the simplest andpreferred manner of performing this function is to employ an electronicswitch as the switch S1 which is cut oif simply by the decrease ofvoltage across its load terminals as the capacitor C1 becomes charged tosome desired value.

At a later time 18, determined by the sequencing circuits SQ, the switchS2 will be again closed, and the capacitor C1 will be discharged throughthe test indicator TK and the diode D2. Operation of the test indicatorTK to produce its indication will indicate both that the circuit isintact from ground through the battery, the switch S1, the bridge wire Wand the diode D1, and that the switch S2 is functioning properly. Shouldthere be a ground in the firing circuit, or if the switch S2 has failedto open, the inhibt signal will be produced, and even if it is notproduced and current is caused to flow through the wire W, breaking it,the capacitor C1 will not be charged and it will be realized thatservice is needed.

As soon as the switch S2 is again closed, at t8, the inhibit level willbe produced to prevent further operation of the sequencing circuits SQ.The apparatus will remain in that condition until the time I10 when thetest command signal is removed, causing the indicator to go back to itsofi state.

FIG. 3 shows a specific embodiment of my invention adapted for use in anexplosion suppression system, as for fuel tank protection in aircraftand the like. A typical installation is designed to protect a set offuel tanks in the wing of an aircraft having vents communicating with asurge tank, the latter being vented through a vent conduit leading to aport on the wing.

The apparatus of FIG. 3 comprises a pair of explosion supporters XS1 andX52 of conventional design, adapted to be mounted in spaced relation inthe wall of a surge tank or other container to be protected. Preferably,the apparatus includes an indicator TK, of any suitable electricallyoperated, visual type, adapted for mounting in an exposed place in whichit will be readily observed, as in the outer surface of an aircraft. Thesignal generator SG comprises a photocell PC, such as a silicon solarcell or the like, to be mounted in position to respond to radiation froma flame front corresponding to an incipient explosion. For example, inthe aircraft protection system just described, the photocell PC wouldpreferably be mounted in the wall of the vent conduit.

The explosion suppressors XS1 and XS2 may be of any conventional type,and may comprise, for example, a frangible steel container of aninerting fluid, such as dibromotetrafluoroethane or the like, and anelectrically detonated charge for bursting the container. Theelectrically detonated charge may comprise an electric blasting cap orthe like. The indicator K may be of the conventional type actuated by acharge ignited by current, in which the force of the explosion is usedto provide the desired indication, as by casting a quantity of brightcolored powder from a concealed position to a position where it isvisible. Such an indicator also serves as a fuse in the circuit,providing an open circuit across its terminals after actuation to giveadditional insurance against voltage appearing across the open terminalsof the suppressor after their actuation. If another form of indicator,or no indication, is provided, a fuse is preferably included in thecircuit.

The firing circuit for indicator K should be substantially the sameelectrically as those of the suppressors XS1 and X82, and include abridge wire W1 inserted in the charge for setting it ofif when currentis applied. As shown, the suppressors XS1 and X52 are provided withsimilar bridge wires W2 and W3 connected in series with the bridge wireW1 of the indicator K. The firing circuit for the suppressors andindicator extends from the positive terminal B+ of the power supply, notshown, such as a 28 volt D.C. source, in series through the switch S1,the bridge wires W1, W2 and W3, the diode D1, and the switch S2 toground in the same manner as described in connection with FIG. 1.

The switch S1 comprises a controlled rectifier SCR1 having its anode andcathode connected in the firing path. The path between the anode andcathode is made conductive by gate current supplied to the gate terminalof the controlled rectifier SCR1 with respect to its cathode.Preferably, a capacitor C2 is connected between the anode of thecontrolled rectifier SCR1 and ground to protect the controlled rectifieragainst false operation by voltage transients in the supply. If desired,the controlled rectifier SCR1 may be replaced by another form ofelectronic switch, such as a power transistor, silicon controlledswitch, or the like, without departing from the scope of my invention inits broader aspects.

The control unit SC1 for the switch S1 comprises a tunnel diode TDconnected between the gate terminal and the cathode of the siliconcontrolled rectifier SCR1. The tunnel diode TD will pass current appliedto it in the forward direction below a background level which it isdesired to ignore, and will snap through its valley region to switchcurrent into the control gate of the silicon controlled recitfier SCR1and gate the latter into conduction when the current supplied to thetunnel diode is above the predetermined background level.

A control circuit for the tunnel diode TD comprises the signal generatorSG including a photovoltaic cell PC. The photocell PC is connectedacross the input terminals of a conventional DC amplifier A, to producea current signal when the photocell PC is irradiated.

The output terminals of the amplifier A are connected between ground anda second terminal connected to the anode of the tunnel diode TD. Thecathode of the tunnel diode TD is returned to ground through threealternate parallel paths. The first comprises the firing circuit for theindicator and suppressors, and extends through the bridge wires W1, W2,and W3 of the indicator and suppressors in series, through the diode D1,and through the switch S2 to ground. As shown, the switch S2 comprises acontrolled rectifier SCR2 having its anode and cathode connected in theseries firing path.

A second path from the cathode of the tunnel diode TD also includes theindicator and suppressors and the diode D1 in series. The second pathextends from the cathode of the diode D1 through a storage capacitor C1,and through the current limiter CL to ground.

A third path extends from the cathode of the tunnel diode TD through atiming network comprising part of the sequencing circuits SQ. Thisnetwork includes a series path including an isolating diode D3 and aresistor R1. The resistor R1 extends to ground through a first pathcomprising a capacitor C7, and through a second path to ground throughan isolating diode D4 and a capacitor C3 connected in parallel with avoltage divider comprising a resistor R2 and a resistor R3 in a series.

The gate terminal of the controlled rectifier SCR2 is returned toground, and the cathode of the controlled rectifier SCR2, through aswitch control circuit SC2 comprising an npn transistor Q1 having itscollector connected to the gate terminal of the controlled rectifierSCR2 and its emitter connected to ground. The collector of thetransistor Q1 is returned to the positive terminal B+ of the powersupply through a resistor R4. The base of the transistor Q1 is connectedto the junction of the resistors R2 and R3 in the sequencing circuitsSQ.

The current limiter CL comprises an isolating diode D5 having its anodeconnected to one terminal of the capacitor C-1, and its cathodeconnected to ground through a resistor R5. A Zener diode Z1 has itsanode connected to the junction of the diode D5 and the resistor R5, andits cathode connected to the test command input terminal. The resistorR5 serves to limit the current flowing through the capacitor C1 duringthe test mode of operation, in a manner to appear, and the Zener diodeZ1 serves to block the diode D5 until the voltage at the cathode of thediode D1 is greater than a value equal to the positive supply voltageless the Zener breakdown drop across the Zener diode Z1. The purpose ofthat arrangement is to prevent the buildup of charge on C1 unless theswitch SCR'l completely c oses.

The indicator TK comprises a lamp L2 controlled by a controlledrectifier SCR3 and an npn transistor Q2. As shown, an energizing circuitfor the lamp L2 extends from the input terminal a of the sequencingcircuits SQ through the filament of the lamp L2, and through theanode-to-cathode path of the controlled rectifier SCR3 to ground. Arelatively small capacitor C4 is connected in parallel with the anodeand cathode of the controlled rectifier SCR3 to protect the controlledrectifier against transients that might jolt it into conduction. Thiscapacitor is chosen to be too small to permit noticeable blinking of thelamp L2 when the test command level is first applied. As shown, the testcommand level may be produced by depression of a pushbutton B connectedto the positive terminal B+ of the power supply.

The gate terminal of the controlled rectifier SCR3 is connected to thecollector of the transistor Q2, and the emitter of the transistor Q2 isconnected to ground and to the grounded cathode of the controlledrectifier SCR3. The collector of the transistor Q2 is returned to thetest command terminal a through a resistor R6. The base of thetransistor Q2 is returned to the positive terminal B+ of the powersupply through a resistor R7, and is connected to ground through a pathincluding the diode D2, the capacitor C1, and the controlled rectifierSCR2 in its conducting state. It will be apparent that with thecontrolled rectifier SCR2 non-conducting, the base emitter path of thetransistor Q2 will be forward-biased by current through the transistorR7. When forward biased,

the transistor Q2 will conduct saturation current be tween its collectorand emitter, holding the gate terminal of the controlled rectifier SCR3substantially at ground potential and thereby preventing conductionthrough its anode-to-cathode path. The lamp L2 is accordinglyde-energized under these conditions even though the test command levelmay be present.

The sequencing circuits SQ include a lamp L1 illuminated at times tosimulate a radiation signal produced by a flame front. In practice, thelamp L1 is mounted adjacent the photocell PC in the signal generator SG,to illuminate the photocell when the lamp L1 is energized. An energizingcircuit for the lamp L1 extends from the input terminal a of thesequencing circuits SQ through the emitter-to-collector path of an npntransistor Q3 and thence through the lamp L1 to ground.

An inhibiting path at times preventing the forwardbiasing of thetransistor Q3 extends from the base of the transistor Q3 through anisolating diode D6 and through the diode D1 and the controlled rectifierSCR2 in its conducting state to ground. The purpose of the inhibitingcircuit is to protect the suppressors against actuation when the testcommand level is produced and the controlled rectifier SCR2 isconducting or shorted. The circuit also protects against operation ofthe lamp L1 in the event that there is a ground in the suppressors.

The base of the transistor Q3 is biased with respect to its emitter by atiming network. This network includes a first voltage divider, extendingfrom the input terminal a of the sequencing circuits SQ to ground, andcomprising in series a resistor R8 and a Zener diode Z2. The breakdownvoltage of the Zener diode Z2 is selected to be a desired level belowthe test command potential B+ convenient for biasing other circuitcomponents in a manner to be described.

The cathode of the Zener diode Z2 is returned to ground through a pathextending through a capacitor C5, a resistor R9, and thence through acapacitor C6 in parallel with a Zener diode Z3 to ground. The Zenerdiode Z3 is selected to produce a voltage drop adequate to bias the baseof the transistor Q3 forward with respect to its emitter by a sufficientamount, such as six volts, to cause the lamp L1 to be lighted. As shown,the base of the transistor Q3 is connected to the anode of the Zenerdiode Z3. It will be apparent that when a test com mand signal level isproduced, the base of the transistor Q3 will rise from ground to theZener breakdown potential of the Zener diode Z3, and then return toground by discharge of the capacitor C6 as the capacitor C5 continues tocharge up to the breakdown potential of the Zener diode Z2. This actionwill produce a flash of light from the lamp L1, applying a simulatedcommand pulse to the photoconductive cell PC.

A second timing network extends from the cathode of the Zener diode Z2through the capacitor C5, a diode D7 and thence to ground through thecapacitor C3 in parallel with the voltage-dividing resistors R2 and R3.

Having described the apparatus of FIG. 3, its operation will next bedescribed. First, consider the state of the apparatus with the testpushbutton TB open and the photocell PC dark. The amplifier A willproduce no output signal, or at most a background noise signal that willbe passed by the tunnel diode TD without triggering the controlrectifier SCRl. The capacitor C2 will be charged to B+. The transistorQ1 will be non-conducting; thus, gate current will be supplied to thecontrolled rectifier SCR2 to hold it in a conducting state. Thecapacitors C1, C3, C7, C5 and C6 will be essentially discharged.

Assuming it is now desired to test the apparatus of FIG. 3, the testcommand pushbutton TB is depressed, causing the level test command toappear, at a potential of B+, at the input terminal a of the sequencingcircuits SQ. The Zener diode Z2 serves to protect the circuit componentsfrom over-voltage.

The capacitor C5 will commence to charge through a first path includingthe resistor R9 .and the capacitor C6, and a second path including thediode D7 and the capacitor C3. Comparing FIG. 2 with FIG. 3, at a timeshortly after the test command level appears, the voltage across theresistor R3 will rise to a value gating the transistor Q1 on, bringingthe gate terminal of the controlled rectifier SCR2 essentially to groundpotential and thereby cutting it off. Thereafter, as the capacitor C6 ischarged, the voltage will rise to the potential of the Zener diode Z3and remain at that level briefiy while the lamp L1 is flashed. That willoccur when the transistor Q3 is both forward-biased and its base is highenough above ground potential to supply sufiicient current to illuminatethe lamp L1. As the capacitor C5 continues to charge, the capacitor C6will discharge through the baseemitter path of the transistor Q3 and thelamp filament L1, and the lamp L1 will go out.

At a time t3 shortly after the lamp L1 has flashed, the photoconductivecell PC will produce an input current to the amplifier A to cause it tosupply current to the tunnel diode. TD. This current will return toground through the diode D3, the resistor R1, and the capacitor C7.Diode D4 will be in its blocked state because of the voltage across thecapacitor C3. The silicon controlled rectifier SCR2 will benon-conducting at this time, and current cannot flow through thecapacitor C1 because the current limiter CL will be in its blocked stateat this time. In one practical embodiment of the invention, positivepotential 13+ was 28 volts and the Zener diode Z1 had a breakdownvoltage of 6 volts, resulting in 22 volts across the resistor R5blocking the diode D5.

As soon as the tunnel diode TD is triggered on, the silicon controlledrectifier SCR1 will be gated into conduction and the current will flowfrom B+ through the bridge wires W1, W2 and W3 of the indicator K andsuppressors X81 and X82, respectively. Current will fiow from the bridgewire W3 through the diode D1 and charge the capacitor C1 through thecurrent limiter CL substantially as soon as the controlled rectifierSCR1 is gated on, because the battery voltage of 28 volts, less thedrops across the indicator and suppressors and the forward gaps of thecontrolled rectifier SCR2 and the diodes D1 and D5, will exceed thevoltage across the resistor R5. However, the current charging thecapacitor C1 will be limited both by the resistor R5 and by the maximumvoltage of 3 or 4 volts that can be applied across the capacitor C1.

As the capacitor C1 is charging, the capacitor C7 is also chargingthrough the controlled rectifier SCR1, the diode D1 and the resistor R1.At a point during the charge of the capacitor C7 as the capacitor C3discharges, the diode D4 will become forward-biased and the voltageacross the resistors R2 and R3 will be held up as long as the controlledrectifier SCR1 is conducting to hold the transistor Q1 in conduction andthus block the controlled rectifier SCR2. As the capacitors C1, C7 andC3 continue to charge, the current through the controlled rectifier SCR1will go so low that conduction will cease, and the controlled rectifierwill not conduct again until again triggered.

When the controlled rectifier ceases to conduct, the capacitors C7 andC3 will discharge through the resistors R2 and R3 for a predeterminedtime, such as two or three seconds. At the end of that time thetransistor Q1 will no longer be forward-biased, and its collector willrise to a positive potential with respect to ground, gating on thecontrolled rectifier SCR2. When the controlled rectifier SCR2 is in itsconducting state, a negative potential is applied by the capacitor C1through the diode D2 to the base of transistor Q2 with respect to itsgrounded emitter. That will momentarily cut off the transistor Q2 whilethe capacitor discharges through the leakage resistance of theemitter-base path and R7. When the tarnsistor Q2 is cut off, the gateterminal of the controlled rectifier SCR3 will rise in potential anddraw gating cur- 8 rent with respect to its cathode, causing theanode-tocathode path of the controlled rectifier SCR3 to becomeconducting and allow the lamp L2 to be illuminated. The lamp L2 willremain illuminated, indicating proper operation of the apparatus, untilthe test pushbutton TB is released. The apparatus will then be in itsstandby state, with the controlled rectifier SCR1 cut ofii and thecontrolled rectifier SCR2 gated on.

It will be apparent that the capacitor C1 can be charged during theinitial portion of the testing cycle only through the actuating bridgewires of the series-connected indicator and suppressors. The limitationon the voltage appearing at the base of the transistor Q3, for example,to 6 volts, makes it impossible for the voltage at that point to chargethe capacitor C1, because the diode D5 and the current limiter C1 isblocked by the Zener diode Z1 until a much higher potential of, forexample, 28 volts, is applied. If the suppressor firing circuit is notintact, the capacitor C1 will not be charged. At the same time, thecontrolled rectifier SCR2 is tested by the fact that the capacitor C1will not charge if the anode-tocathode path of the controlled rectifieris shorted to ground or fails to open electrically. A short circuit toground in the suppressor and indicator line will cause the operation ofthe lamp L1 to be inhibited, also preventing the charging of thecapacitor C1. Finally, when a charge is stored on the capacitor C1 theconducting state of the controlled rectifier SCR2 is tested by requiringit to be in a conducting state before the lamp L2 can be turned on.

Operation of the apparatus of FIG. 3 in its explosion suppressing modeof operation will not be affected by the testing and simulatingcircuits. If the photocell PC is illuminated by radiation signalling anincipient explosion, the tunnel diode TD will trigger the controlledrectifier SCR1 into conduction and supply a pulse of actuating currentto the bridge wires of the indicator and suppressors, and thence toground through diode D1 and the controlled rectifier SCR2. Thecontrolled rectifier SCR1 will be gated ofi .as soon as the bridge wiresW1, W2 and W3 are broken, preventing any application of voltage to theterminals of these devices after their actuation. In order to avoidinterference with this desirable feature, the resistance to ground fromthe cathode is made sufficiently high that the controlled rectifier SCR1cannot be gated into steady conduction, because the high resistance toground from the cathode of the controlled rectifier SCR1 limits itscurrent to a value too low to permit conduction to be sustained. By thatarrangement, retriggering of the controlled rectifier SCR1 is preventedafter the suppressors have been operated.

While I have described my invention with respect to the details of apreferred embodiment thereof, many changes and variations will becomeapparent to those skilled in the art upon reading my description, andsuch can obviously be made without departing from the scope of myinvention. In particular, while I have described a specific embodimentparticularly adapted for use as a fuel tank protection system, it iscontemplated that the apparatus of my invention in its broader aspectscan be applied to the non-destructive testing of other circuitsincluding current-interrupted circuit elements.

Having thus described my invention, what I claim is:

1. Condition simulating and circuit testing means for a control system;said control system comprising first signal generating means forproducing an electrical signal in response to a predetermined inputsignal and a series circuit comprising a source of voltage, a firstswitch closed by said electrical signal, and control means comprising aconductor connected between a pair of terminals in said circuit, saidconductor conducting current below a predetermined value and beingbroken by current above said predetermined value, in which said sourcesupplies current sufiicient to interrupt said conductor when said firstswitch is closed; said simulating and testing means comprising a secondnormally closed switch in said series circuit, a capacitor connected inparallel with said second switch, current limiting means connected inseries with said capacitor to limit the flow of current therethrough toa value below said predetermined value, system test signal generatingmeans for producing a test signal, means responsive to said test signalfor opening said second switch, first time delay means responsive tosaid test signal for producing a simulating signal a predetermined timeafter said second switch is opened, means responsive to said simulatingsignal for applying an input signal to said first signal generatingmeans to thereby cause said first switch to be closed to charge saidcapacitor through said conductor with a current below said predeterminedvalue, means for opening said first switch when said capacitor ischarged to a predetermined voltage, signal generating means connected inseries with said capacitor and said second switch for producing anoutput signal indicating proper operation of said system when saidsecond switch is closed and discharge current flows from said capacitor,and second time delay means responsive to the opening of said firstswitch for closing said second switch a predetermined time after saidfirst switch is opened to restore the system to normal operation,

2. The apparatus of claim 1, in which said first signal generating meanscomprises radiation detecting means for producing a current signal inresponse to radiation from a flame, and said conductor comprises thebridge wire of an electrically actuated explosion suppressor.

3. Apparatus for the non-destructive testing of an electrical circuit ofthe kind comprising a circuit element interrupted by the passage ofcurrent above a predetermined value and connected in series with asource of direct voltage and a normally open first switch, in whichclosing the first switch will cause the flow of current above saidpredetermined value in the circuit element, said ap paratus comprising asecond normally closed switch connected in series with said circuitelement, a capacitor and current limiting means connected in a firstseries combination, said series combination being connected in parallelwith said second switch, a rectifier and a voltage indicator connectedin series with said capacitor in a second series combination, saidsecond series combination being connected in parallel with said secondswitch, said rectifier being poled to oppose the flow of current fromsaid source, said voltage indicator comprising means for producing anoutput signal when said capacitor is charged by said source and saidsecond switch is closed, means for producing a test command signal,means controlled by said test command signal for opening said secondswitch, first time delay means responsive to said command signal forclosing said first switch after said second switch is opened to chargesaid capacitor through said circuit element and said current limitingmeans, said current limiting means holding the charging current belowsaid predetermined value, means responsive to the charging of saidcapacitor for opening said first switch when a predetermined charge hasbeen stored in the capacitor, second time delay means enabled by theclosing of said first switch and responsive to the opening of said firstswitch to close said second switch a predetermined time after said firstswitch is opened and thereby discharge said capacitor through saidrectifier and said voltage indicator, whereby said indicator produces anoutput signal only when the circuit to be tested is operative.

4. An explosion suppression system for protecting a container at leastpartially enclosing a space that may contain a combustible gas againstan explosive rise of pressure following ignition of gas, comprising anexplosion suppressor mounted in the container and having a bridge wireand an explosive charge ignited by current above a predetermined valueflowing in the bridge wire and breaking the wire, a radiation detectormounted in position for radiation from a flame front propagating in thecontainer to produce a first output signal, a first normally openelectronic switch, a second normally closed electronic switch, a sourceof direct voltage connected in series with said switches and said bridgewire, means responsive to said first output signal for closing saidfirst switch, a capacitor and current limiting means connected in afirst series combination, said first combination being connected inparallel with said second switch, a rectifier and a voltage indicatorconnected in series with said capacitor in a second series combination,said second combination being connected in parallel with said secondswitch, said rectifier being poled to oppose the flow of current fromsaid source, said voltage indicator comprising means for producing asecond output signal when said capacitor is charged by said source andsaid second switch is closed, means for producing a test command signal,means controlled by said command signal for opening said second switch,flame simulating means responsive to an applied signal to irradiate saidradiation detector and cause it to produce an output signal, first timedelay means responsive tosaid command signal to apply a signal to saidflame simulating means to cause said first switch to close apredetermined time after said second switch is opened to charge saidcapacitor through said bridge wire and said current limiting means, saidcurrent limiting means holding the charging current below saidpredetermined value, means responsive to the charging of said capacitorfor opening said first switch when a predetermined charge has beenstored in the capacitor, second time delay means enabled by the closingof said first switch and responsive to the opening of said first switchto close said second switch a predetermined time after said first switchis opened and thereby discharge said capacitor through said rectifierand said voltage indicator, whereby said indicator produces an outputsignal only when the circuit to be tested is operative, and allcomponents of the suppressor firing circuit are tested for properoperation.

References Cited UNITED STATES PATENTS THOMAS B. HABECKER, PrimaryExaminer.

U.S. Cl. X.R.

1.CONDITION SIMULATING AND CIRCUIT TESTING MEANS FOR A CONTROL SYSTEM;SAID CONTROL SYSTEM COMPRISING FIRST SIGNAL GENERATING MEANS FORPRODUCING AN ELECTRICAL SIGNAL IN RESPONSE TO A PREDETERMINED INPUTSIGNAL AND A SERIES CIRCUIT COMPRISING A SOURCE OF VOLTAGE, A FIRSTSWITCH CLOSED BY SAID ELECTRICAL SIGNAL, AND CONTROL MEANS COMPRISING ACONDUCTOR CONNECTED BETWEEN A PAIR OF TERMINALS IN SAID CIRCUIT, SAIDCONDUCTOR CONDUCTING CURRENT BELOW A PREDETERMINED VALUE AND BEINGBROKEN BY CURRENT ABOVE SAID PREDETERMINED VALUE, IN WHICH SAID SOURCESUPPLIES CURRENT SUFFICIENT TO INTERRUPT SAID CONDUCTOR WHEN SAID FIRSTSWITCH IS CLOSED; SAID SIMULATING AND TESTING MEANS COMPRISING A SECONDNORMALLY CLOSED SWITCH IN SAID SERIES CIRCUIT, A CAPACITOR CONNECTED INPARALLEL WITH SAID SECOND SWITCH, CURRENT LIMITING MEANS CONNECTED INSERIES WITH SAID CAPACITOR TO LIMIT THE FLOW OF CURRENT THERETHROUGH TOA VALVE BELOW SAID PREDETERMINED VALUE, SYSTEM TEST SIGNAL GENERATINGMEANS FOR PRODUCING A TEST SIGNAL, MEANS RESPONSIVE TO SAID TEST SIGNALFOR OPENING SAID SECOND SWITCH, FIRST TIME DELAY MEANS RESPONSIVE TOSAIDD TEST SIGNAL FOR PRODUCING A SIMULATING SIGNAL A PREDETERMINED TIMEAFTER SAID SECOND SWITCH IS OPENED, MEANS RESPONSIVE TO SAID SIMULATINGFOR APPLYING AN INPUT SIGNAL TO